A vehicle body component (vehicle framework component such as members, pillars, or a floor tunnel, for example) of an automobile can be fabricated by press forming a planar workpiece made of metal to have a hat-shaped cross section.
FIG. 1A and FIG. 1B are schematic views showing an example of a product with a hat-shaped cross section, FIG. 1A being a side view and FIG. 1B being a I-I cross-sectional view of FIG. 1A. The product 10 with the hat-shaped cross section has a web portion 10a, a first vertical wall portion 10b extending toward one side of the web portion 10a, a first flange portion 10c further extending from the first vertical wall portion 10b, a second vertical wall portion 10b extending toward the other side of the web portion 10a, and a second flange portion 10c further extending from the second vertical wall portion 10b. 
In the example of FIG. 1A and FIG. 1B, the web portion 10a of the product 10 is provided with inclinations in a front section and a rear section in a longitudinal direction. As described above, in the product 10, there is a case where a total of section line lengths substantially changes in the longitudinal direction. Here, the “total of section line lengths” is obtained by adding each section line length of the web portion 10a, the first and second vertical wall portions 10b, and the first and second flange portions 10c. 
Further, with regard to the vertical wall portions 10b of the product 10, one is vertical to the flange portion 10c and the other is inclined from a state of being vertical to the flange portion 10c, inclinations being different in both sides. As described above, there is a case where a cross-sectional shape of the product 10 is asymmetric.
FIG. 2A and FIG. 2B are schematic views showing an example of a product with a hat-shaped cross section, FIG. 2A being a side view and FIG. 2B being a II-II cross-sectional view of FIG. 2A. Note that the same reference numeral is granted to a component the same as that of the product with the hat-shaped cross section of FIG. 1A and FIG. 1B and explanation thereof will be omitted.
In FIG. 2A and FIG. 2B is shown an example in which a web portion 10a of a product 10 is provided with inclinations in a front section and a rear section in a longitudinal direction, a lower surface of the product 10 is also similarly provided with inclinations, and a total of section line lengths does not change in a longitudinal direction. Further, the above is the example in which a cross-sectional shape of the product 10 is symmetric.
FIG. 3A and FIG. 3B are schematic views showing an example of a product of a hat-shaped cross section, FIG. 3A being a plan view and FIG. 3B being a cross-sectional view of FIG. 3A. Note that the same reference numeral is granted to a component the same as that of the product with the hat-shaped cross section of FIG. 1A and FIG. 1B, and explanation thereof will be omitted.
In FIG. 3A and FIG. 3B is shown the example in which an entire product 10 is bent unilaterally.
Note that though the flange portion 10c is provided along an entire length in the longitudinal direction in the product 10 with the hat-shaped cross section shown in FIG. 1A, FIG. 1B, FIG. 2A, FIG. 2B, FIG. 3A, and FIG. 3B, there are cases where a flange portion 10c is provided in a part of a longitudinal direction.
The product 10 with the hat-shaped cross section as above can be fabricated by press forming in which a punch and a die are used, and at that time, a pad is used together with a blank holder.
FIG. 4A to FIG. 4C are cross-sectional views schematically showing a processing flow of a case where a pad is used together with blank holders in press forming, FIG. 4A showing a sandwiching and binding time of a workpiece, FIG. 4B showing a pushing time, and FIG. 4C showing a starting time of separating operation of the punch and the die after a bottom dead center is reached, respectively. In FIG. 4A to FIG. 4C, a case is shown where a web portion 10a, vertical wall portions 10b, and flange portions 10c are formed in the workpiece when an upper forming tool is lowered from a top dead center to the bottom dead center.
FIG. 4A to FIG. 4C show the workpiece 50 and a forming tool 20 which a press forming apparatus has. In the metal forming tool 20, the upper forming tool 40 is constituted with the die 41 and the pad 42 disposed to face the punch 31, and a lower forming tool 30 is constituted with the punch 31, the blank holders 32 disposed adjacent to the punch 31.
The punch 31 of the lower forming tool 30 has a tip surface 31a having a shape corresponding to the web portion 10a, and an outside surface 31b having a shape corresponding to the vertical wall portions 10b. On the other hand, the die 41 of the upper forming tool 40 has a recessed portion and an inside surface 41a thereof has a shape corresponding to the outside surface 31b of the punch.
Further, the pad 42 is disposed in the recessed portion of the die 41, and the pad 42 is mounted on the die 41 via a pad pressure mechanism (for example, a spring or a gas cylinder) 43. The pad 42 mounted as above is slidable in a pressing direction. A tip surface (a surface facing the tip surface 31a of the punch) of the pad 42 has a shape corresponding to the tip surface 31a of the punch. On the other hand, the blank holders 32 are disposed on both sides of the punch 31, and the blank holder 32 is supported slidably in the pressing direction by a blank holder pressure mechanism (for example, a spring, a hydraulic cylinder, or a gas cylinder) 33. Here, the pressing direction means a direction where the punch 31 and the die 41 relatively move at a time of press forming, and in the metal forming tool 20 shown in FIG. 4A to FIG. 4C, a vertical direction is the pressing direction.
In press forming using the metal forming tool 20 with such a configuration, the planar workpiece 50 made of metal is disposed between the die 41 and the punch 31. When the upper forming tool 40 is lowered from the top dead center in such a state, the pad 42 abuts on the workpiece 50 and the pad pressure mechanism 43 is compressed. Thereby, the pad 42 is pressed to the workpiece 50 by a restoring force of the pad pressure mechanism 43, and as shown in FIG. 4A, a middle portion in a width direction of the workpiece 50 is sandwiched by the pad 42 and the punch 31.
Further, the die 41 abuts on the blank holder 32 via the workpiece 50 and the blank holder pressure mechanism 33 is compressed. Thereby, the blank holder 32 is pressed to the workpiece 50 by a restoring force of the blank holder pressure mechanism 33, and an outer peripheral portion in the width direction of the workpiece 50 is bound by the blank holder 32 and the die 41.
Here, timings of sandwiching the middle portion by the pad 42 and the punch 31 and of binding the outer peripheral portion by the blank holder 32 and the die 41 are properly set in corresponding with a shape or the like of the product. For example, there is a case where sandwiching the middle portion by the pad 42 and the punch 31 is carried out simultaneously with binding the outer peripheral portion by the blank holder 32 and the die 41. Further, there is a case where the outer peripheral portion is bound by the blank holder 32 and the die 41 after the middle portion is sandwiched by the pad 42 and the punch 31. Further, there is a case where the middle portion is bound by the pad 42 and the punch 31 after the outer peripheral portion is sandwiched by the blank holder 32 and the die 41.
In a state where the middle portion in the width direction of the workpiece 50 is sandwiched and the outer peripheral portion is bound, the upper forming tool 40 is further lowered as shown in FIG. 4B. Thereby, the punch 31 and the die 41 are relatively moved and the workpiece 50 is pushed into the recessed portion of the die 41 together with the punch 31 to draw the workpiece 50, whereby press forming is carried out.
At a time of press forming, since the workpiece 50 is pushed into the recessed portion of the die 41 together with the punch 31, both end positions of the workpiece 50 move toward the recessed portion of the die 41. Therefore, a binding length L of the workpiece 50 bound by the blank holder 32 and the die 41 becomes shorter as press forming progresses (see FIG. 4A and FIG. 4B).
Then, when the upper forming tool 40 reaches the bottom dead center, the web portion 10a is shaped by the tip surface 31a of the punch 31 and the pad 42, and the vertical wall portion 10b is shaped by the outside surface 31b of the punch 31 and the die 41. Further, the flange portion 10c is shaped by the blank holder 32 and the die 41. Consequently, the workpiece 50 is formed to have a hat-shaped cross section. As a result that the upper forming tool 40 is raised in a state where the upper forming tool 40 reaches the bottom dead center as shown in FIG. 4C, the punch 31 and the die 41 relatively move and are separated.
In such press forming using the pad 42 together with the blank holder 32, the middle portion in the width direction of the workpiece 50 is sandwiched by the pad 42. Thereby, in a process of forming the web portion 10a, the vertical wall portion 10b, and the flange portion 10c in the workpiece 50 (hereinafter, also simply referred to as a “forming process”), in a case where a shape of a product is an asymmetric shape, for example, there can be reduced occurrence of an unintended excessive displacement in the web portion 10a in a case where inclinations of the vertical wall portions 10b are substantially different in both sides. Further, by an effect of sandwiching and binding by the pad 42 and the blank holder 32, there can be reduced formation of a wrinkle in the web portion 10a, the vertical wall portion 10b, the flange portion 10c, or near a boundary between the web portion 10a and the vertical wall portion 10b, or in the periphery of a ridge line portion of the flange portion 10c and the vertical wall portion 10b, and sandwiching and binding by the pad 42 and the blank holder 32 is particularly effective in forming a product having a shape in which a total of section line lengths substantially changes in a longitudinal direction, or a shape which is bent in a side view or a top view (see FIG. 2 and FIG. 3).
In press forming using a pad together with a blank holder, there are a case of using a pad which abuts on the entire of a web portion 10a to be formed and a case of using a pad which abuts on a part of a web portion 10a to be formed, as the pad. In the former case, the web portion 10a is shaped by a tip surface of a punch and the pad, and in the latter case, the web portion 10a is shaped mainly by a tip surface of a punch, the pad, and a die.
Further, the die 41 is sometimes constituted with a single member as shown in FIG. 4A to FIG. 4C, and is sometimes constituted with a plurality of members as shown in FIG. 5.
FIG. 5 is a cross-sectional view schematically showing a sandwiching and binding time of a workpiece of press forming using a die constituted by the plurality of members. FIG. 5 shows the workpiece 50 and a metal forming tool 20 which a press forming apparatus has. The metal forming tool 20 has a basic configuration the same as that of the metal forming tool shown in FIG. 4A to FIG. 4C, but the die constituted with the plurality of members is used as the die 41 having a recessed portion. The die 41 is constituted with a first block 411 forming one inside surface 41a of the recessed portion, a second block 411 forming the other inside surface 41a of the recessed portion, and a plate 412 fixing the first block 411 and the second block 411. A pad 42 is mounted on the plate 412 via a pad pressure mechanism 43.
The product with the hat-shaped cross section having been press formed is properly subjected to a processing such as trimming and is shaped into a finished product. On that occasion, there is a case where the flange portions 10c are removed, by cutting or the like, from the product with the hat-shaped cross section so that a finished product constituted with the web portion 10a, the first vertical portion 10b, and the second vertical wall portion 10b is made.
Press forming using the pad together with the blank holder is also applicable to a case where a flange portion 10c is provided in a part of a longitudinal direction in a product with a hat-shaped cross section, by properly altering a shape of a workpiece. Further, press forming using the pad together with the blank holder is applicable not only to a case of the hat-shaped cross section but also to a case of press forming a product in which a flange portion 10c is provided in only one side. To the product in which the flange portion 10c is provided in only one side, there corresponds a product constituted with a web portion 10a, a first vertical wall portion 10b, a second vertical wall portion 10b, and a first flange portion 10c. Alternatively, there corresponds a product constituted with a web portion 10a, a first vertical wall portion 10b, a second vertical wall portion 10b, and a second flange portion 10c. 
Here, in press forming using the pad together with the blank holder, there is an apprehension that a deformation occurs in the workpiece when the punch and the die are separated after shaping of the workpiece into a predetermined shape. When the punch 31 and the die 41 are separated, since the blank holder 32 is pressed to the workpiece 50 in order to bind the workpiece 50, the flange portion 10c of the workpiece 50 is pressed in a direction shown by a dashed line arrow in FIG. 4C by the blank holder 32. On the other hand, since the pad 42 is pressed to the workpiece 50 in order to sandwich the workpiece 50, the web portion 10a of the workpiece 50 is pressed in a direction shown by a solid line arrow in FIG. 4C by the pad 42. Consequently, the formed workpiece 50 is deformed.
In order to prevent the workpiece from being deformed when the punch and the die are separated as described above, a locking mechanism has been conventionally used. In a case of a vehicle body component of an automobile, in particular, since a pressing force at a time of sandwiching and binding a workpiece by a pad and a blank holder is normally quite large, 2 tonf or more, a deformation of the workpiece at a time of separating a punch and a die being inevitable, a locking mechanism is essential.
With regard to press forming using the pad together with the blank holder, various suggestions have been conventionally made, such as by Patent Literature 1 and Patent Literature 2, for example.
In Patent Literature 1, there is suggested a drawing forming tool which has a locking mechanism temporarily binding a pad (cushion) in a lower forming tool side at a time of separating a punch of an upper forming tool and a die of a lower forming tool. The locking mechanism of the drawing forming tool is constituted with a cam driver, a parent cam member, and a child cam member, and the pad of the lower forming tool is mechanically bound to delay a timing of rising. Thereby, it is said that compared with a case where binding of a pad is controlled by hydraulic pressure, a deviation of timing can be prevented and that a deformation of a workpiece can be prevented.
Further, in Patent Literature 2, there is suggested a metal forming tool a lower forming tool of which is constituted with a punch and a blank holder, an upper forming tool of which is constituted with a die and a pad, and which has a locking mechanism built-in. That locking mechanism binds the pad to the die when the die reaches a bottom dead center between the pad and the die, and releases binding of the pad to the die when the die is raised by a predetermined length after passing through the bottom dead center. In an example thereof, a cam is used for materializing the locking mechanism. It is said that such a locking mechanism of Patent Literature 2 can prevent a deformation of a workpiece and can heighten a productivity by being applied to a transfer press apparatus.